Insulating media in high voltage transformers consists of paper wrapped around the
conductors in the transformer coils plus mineral oil and pressboard to insulate the coils from ground. From the moment a transformer is placed in service, both the solid and liquid insulation begin a slow but irreversible process of degradation.
Solid Insulation — Paper
Paper and pressboard are composed primarily of cellulose, which is a naturally occurring polymer of plant origin.
Liquid Insulation — Oil
The insulating fluid that has the greatest use in electrical equipment is mineral oil. There are insulating materials that may be superior to mineral oil with respect to both dielectric and thermal properties; however none has achieved the requisite combination.